The 2002 Pop Idol winner trained in three continents for the marathon - in Australia, North America and Africa.

He told Channel 4 News: "I know it will be gruelling, but thinking about the young people I have met and the seemingly impossible situations they overcome with the support of Catch22 will help me reach the finish."

The disaster led to the cancellation of the Nagoya Marathon, which was to be the final selection race for Japan's World Championships marathon team. London's organisers offered last-minute places in the race.

Former Tokyo marathon winner Mizuho Nasukawa revealed that a school friend, Taiko Osawa, and one of her daughters had been killed in the tsunami on 11 March.

She said: "My spirit was completely broken and I was shocked when I heard my friend passed away. We were team-mates in high school doing track and field. I realised coming to London and showing people through my running would be the best way to remember my friend."

(Pictured: Will Young trains with young people from Catch22)

Raoul Moatpoliceman'mental struggle'

And policeman David Rathband, who was blinded by gunman Raoul Moat, is competing in aid of the Blue Lamp Foundation, which he set up to support people in the emergency services injured in the line of duty by a criminal act. PC Rathband said his preparation had been a mental struggle as well as a physical test.

He said: "It's a very lonely place when you're training.

"I've been to some very, very dark places, and I've had some visitors that I'd choose not to have. But they pop along and I try to run them out, and I've managed to do that.

"But it's public knowledge that the trial went on for a very long time and it ripped my wife and myself apart for the second time, and I'm still trying to rebuild my strength, energy and health."